The use of thermal energy to destroy bodily tissue can be applied to a variety of therapeutic procedures, including the treatment of cardiac arrhythmias, such as atrial fibrillation. In such a procedure, thermal energy can be imparted to the arrhythmogenic myocardium using various forms of energy, such as radio frequency electrical energy, microwave or light wave electromagnetic energy, or ultrasonic vibrational energy. Radio frequency (RF) ablation, for example, can be effected by placing a catheter within the heart and pressing an emitting electrode disposed on the catheter against the heart wall near the region of the myocardium that is causing the arrhythmia. High frequency electrical current can be passed into the tissue between closely spaced emitting electrodes or between the emitting electrode and a larger, common electrode located remotely from the tissue to be heated. The energy can heat the myocardium to a temperature that will cause necrosis (e.g., a temperature above about 50° C.).
One embodiment of a prior art ablation catheter is shown in FIG. 1. The catheter 100 includes a plurality of sensing electrodes 102 disposed thereon that are used to detect electrical activity in the heart. The measurement of electrical activity can be used to detect the arrhythmogenic tissue and guide the placement of the catheter. The catheter also includes a large electrode or other ablation element 104 disposed on the distal end thereof that is effective to transmit RF electrical energy into the myocardium 106. In use, the electrode 104 on the distal tip of the catheter 100 is placed against the surface of the myocardium 106 in a desired location, and the electrode is subsequently activated to heat the tissue.
Prior art ablation catheters have a number of disadvantages. For example, using the above techniques, maximum heating often occurs at or near the interface between the catheter electrode 104 and the tissue 106. In RF ablation, for example, maximum heating can occur in the tissue immediately adjacent to the emitting electrode. Furthermore, as these techniques are increasingly used in areas having thicker tissue walls, the RF power level must be increased to effect heating at greater depths within the tissue. This can result in even higher levels of heating at the interface between the electrode and the tissue. As described in more detail below, these high levels of heating can reduce the conductivity of the tissue, effectively preventing the transmission of further energy into the tissue. In addition, some levels of heating can produce dangerous medical complications for a patient, including, for example, clots that can result from overheating surrounding blood.
Accordingly, there is a need for improved methods and devices for controlling ablation therapy.